A polyvinyl chloride (PVC) resin is a polymer containing 50% or more of vinyl chloride, which is widely utilized in a variety of fields, because it is cheap, hardness thereof is easily controlled and it is applicable to most processed apparatuses and provides molded articles with superior physical and chemical properties.
However, because a polyvinyl chloride resin has poor impact strength, workability, thermal stability and heat deflection temperature, additives to address these problems are developed. For example, additives such as impact modifiers, processing aids, stabilizers and fillers are suitably selected according to applications thereof.
In recent years, interest in foam molding as a method for reducing weight of polyvinyl chloride resins and lowering price of molded articles has increased. For reference, when polyvinyl chloride is foam-molded alone, there are disadvantages in that sufficient elongation and melt strength cannot be obtained and appearance of molded articles is thus bad, and foam cells are large and non-uniform and foaming magnitude is thus low.
Accordingly, in order to solve these disadvantages, addition of a mixture of a high-molecular weight acrylate processing aid containing methyl methacrylate as a predominant component and a foaming agent, to polyvinyl chloride resins was suggested.
For example, U.S. Pat. No. 6,391,976 discloses use of methyl methacrylate and an acrylate monomer as a comonomer thereof and U.S. Pat. No. 6,221,966 discloses preparation of a processing aid by polymerization of a latex polymerized using alkyl acrylate as a major monomer, with methyl methacrylate.
On the other hand, high-molecular weight styrene-acrylonitrile copolymers have competitive price as compared to high-molecular weight acrylate processing aids, but disadvantageously causes deterioration in qualities upon foaming of polyvinyl chloride resins.